The present invention relates to a catalytic alkylation process and apparatus. In a more specific aspect, the present invention relates to a catalytic alkylation process in which an alkylatable hydrocarbon is contacted with an alkylating agent in the presence of an acid-type catalyst and the catalyst is cyclically circulated through the system. In yet another aspect, the present invention relates to a catalytic alkylation process and apparatus in which an acid-type catalyst is cyclically circulated through the system to produce an alkylate product of improved octane number.
Numerous processes are known in the prior art for alkylating an alkylatable hydrocarbon with an alkylating agent in the presence of a catalyst. From a commercial standpoint, the most prevalent systems involve the cyclic circulation of an acid-type catalyst, such as hydrofluoric acid, sulfuric acid, etc. through a reaction zone, a separation zone, a cooling zone and back to the reaction zone.
One such process for the alkylation of hydrocarbons, utilizing the cyclic circulation of the catalyst, introduces the reactant hydrocarbons in a gaseous phase. In this system the gaseous hydrocarbon feed mixture is introduced at a high velocity to the lower portion of the reaction zone and into a continuous catalyst phase maintained in the reaction zone. Suitable conditions of temperature and residence or contact time are provided in the reaction zone whereby the alkylatable hydrocarbon is alkylated. A stream comprising catalyst, alkylate product and unconsumed reactants passes from the upper portion of the reaction zone into a settling zone, wherein separation occurs between the alkylate product phase and the catalyst phase. The alkylate product phase is withdrawn from the settling zone for further processing, as by fractionation, and the catalyst phase is passed downwardly to a cooling zone and thence back into the reaction zone.
Another process, based on the cyclic circulation of the catalyst, which overcomes certain of the inherent deficiencies of the gas phase process and permits operation at low reaction temperatures, introduces the reactant hydrocarbons in the liquid phase. In this process, the hydrocarbon reactants are introduced into the bottom of the reaction zone through constricted passageways, thereby creating a liquid lift system, whose motive power comes from the difference in density of the flowing streams and to some extent from the kinetic energy of the inlet hydrocarbon stream, and at the same time creating small droplets of reactants having a high interfacial area which result in a desirably high reaction rate. The reactant hydrocarbons and catalyst move upwardly through an elongated, tube-type reaction zone, the effluent, including alkylate product, catalyst and unreacted hydrocarbons, is discharged from the reaction zone into a settler-surge vessel, an alkylate product phase is withdrawn for further processing and a catalyst phase is passed downwardly through a tube-type conduit to a cooler and thence back into the reaction zone.
It has generally been recognized in the art that in order to obtain an alkylate product of maximum octane number, the weight percent total acidity of the catalyst should not exceed a given amount. While the tolerable acidity of the catalyst will vary depending upon the reactant hydrocarbons and the temperature of operation, it is generally thought that the total acidity of the catalyst under any conditions should not be higher than about 90 percent, that alkylate products of highest octane number are obtained at acidities substantially lower than this and, consequently, that the weight percent total acidity of the catalyst for optimum octane number should be between about 87 percent and 67 percent. Consequently, the catalyst is in some way diluted so as to maintain the desired total acidity. Obviously, water would be the logical diluting agent to utilize. However, the presence of excessive amounts of water, in systems utilizing acid catalysts, creates problems in that, in conjunction with the acid, water is highly corrosive to the alkylation system and catalyst handling system. Therefore, the acid catalyst is normally utilized in an essentially anhydrous condition. There is a tendency in alkylation systems for water to accumulate as the result of its presence in the hydrocarbon reactants and its absorption from the atmosphere. Therefore, alkylation systems employing acid-type catalysts normally include a so-called catalyst "rerun" system through which catalyst is at least periodically passed in order to remove water therefrom. The rerun system will normally involve withdrawing a portion of the catalyst phase as it flows from the separator to the cooler. The withdrawn catalyst phase is heated to a temperature sufficient to vaporize residual alkylate, unreacted hydrocarbons, and the major part of the acid phase from the water. Water is withdrawn as a bottoms product from the rerun separator while the vapor phase is recycled to the settler-surge zone or back to the separated acid phase. Generally, a portion of the alkylatable hydrocarbon, in liquid form, is utilized as a reflux and another portion, in gaseous form, as a stripping medium in the rerun separator.
It is also recognized in the art that a certain amount of dilution of the catalyst phase occurs in the reaction zone itself. Specifically, what is known as a catalyst- or acid-soluble oil (ASO) is produced in the reaction zone, which inherently acts as a diluent for the catalyst phase. While the specific nature of the acid-soluble oil has not been completely established, it is generally accepted that it comprises predominantly polymeric materials with minor amounts of complexes and small or trace amounts of impurities, such as sulfur, to the extent such impurities exist in the alkylation feed materials. These catalyst-soluble oils are retained in the catalyst phase during the separation of the alkylate phase from the catalyst phase and consequently are recycled to the alkylation system. It has also been universally accepted, by those skilled in the art, that the production of catalyst-soluble oil is substantially in excess of that necessary or desirable for dilution of the catalyst. Consequently, the prior art contains suggestions for the removal of the excess catalyst-soluble oil.
Finally, those skilled in the art have recognized the fact that under normal operating conditions the production of catalyst-soluble oils is extremely slow. Accordingly, the prior art has suggested various start-up procedures which will rapidly produce the desirable inventory of catalyst-soluble oil, thereby substantially shortening the time necessary to arrive at full-scale production of alkylate product.
In contrast to the teachings of the prior art, it has now been found that the amount of catalyst diluent which will produce an alkylate product of maximum octane number is within a relatively narrow range below the amounts heretofore suggested by the prior art. Further, it has been found that a very small change in the amount of catalyst diluent, within the narrow range referred to above, has a substantial effect on the octane number of the alkylate product. Also contrary to the teachings of the prior art, it has been found that not all hydrocarbon reactants produce catalyst-soluble oil at the same rate or in the same ultimate volumes. Specifically, it has been found that when reacting an alkylatable hydrocarbon, such as isobutane, with a C.sub.4 olefinic hydrocarbon, such as butylene, butene-1 and/or butene-2's, catalyst-soluble oils are produced at an extremely slow rate and in relatively small amounts, as compared with other reaction systems. In addition, as the alkylation reaction proceeds, there is a certain attrition of the amount of catalyst-soluble oils in the catalyst phase. While the reasons for this attrition are not fully known, it is believed that a certain amount of the catalyst-soluble oil is carried over with the alkylate product during separation, a greater portion is removed from the system along with water when the catalyst is rerun to remove water and small amounts may actually be consumed during the alkylation reaction. More specifically, it has been found that when an isoparaffin, such as isobutane, is reacted with a C.sub.4 olefinic hydrocarbon, such as butylene, butene-1, and/or butene-2's, an inordinately long period of time is necessary for starting up the alkylation system and there is normally a net loss of catalyst-soluble oil during the course of the reaction. On the other hand, monoolefins, such as C.sub.3 and C.sub.5 and higher molecular weight hydrocarbons, and olefins, such as pentadiene and butadiene, product catalyst-soluble oil in larger amounts and more rapidly.
It would therefore be highly desirable to provide means for substantially shortening the necessary start-up period and for thereafter maintaining a predetermined amount of catalyst-soluble oil in the catalyst system.
It is therefore an object of the present invention to provide an improved system for the alkylation of hydrocarbons. Another and further object of the present invention is to provide an improved system for the alkylation of hydrocarbons, utilizing a cyclic flow of an acid-type catalyst. A further object of the present invention is to provide an improved system for the alkylation of hydrocarbons, utilizing a cyclic flow of an acid-type catalyst, in which an alkylate product of improved octane number is obtained. Yet another object of the present invention is to provide an improved system for the alkylation of hydrocarbons, utilizing a cyclic flow of an acid-type catalyst, in which the acid-soluble oil content of the catalyst phase is maintained within a predetermined critical range. Another and further object of the present invention is to provide an improved system for the start-up of a system for the alkylation of hydrocarbons, utilizing cyclic flow of an acid-type catalyst. These and other objects and advantages of the present invention will be apparent from the following detailed description of the